fix handling of --user and --group options

install-manifest

@@ -27,6 +27,8 @@ static_group_map = {
 
 def _add_to_map(idmap):
     def _add_to_map_fn(option, opt, value, parser):
+        if not value:
+            return
         try:
             username, userid = value.split(':', 1)
         except ValueError:
@@ -180,11 +182,11 @@ metadata.
     parser.add_option('-C', '--cd', '--change-dir', metavar='PATH', dest='cd',
                       help='consider source paths in the MANIFEST as relative to this path')
     parser.add_option('-u', '--user', action='callback', metavar='NAME:ID',
-                      callback=_add_to_map(static_user_map),
+                      callback=_add_to_map(static_user_map), type='str',
                       help='override name -> id mapping in target user map'
                       ' (may be specified multiple times)')
     parser.add_option('-g', '--group', action='callback', metavar='NAME:ID',
-                      callback=_add_to_map(static_group_map),
+                      callback=_add_to_map(static_group_map), type='str',
                       help='override group -> id mapping in target group map'
                       ' (may be specified multiple times)')
     parser.add_option('-n', '--dry-run', dest='dry_run', action='store_true',